The blood of Faramir but not Boromir?

[Mlo]

Ok, this has been bothering me for a while. This is what Gandalf says to Pippin about Denethor.

He is not like other men of his time, Pippin. And whatever be his descent from father to son, by some chance the blood of Westernesse runs nearly true through him; as it does in his other son, Faramir, and yet not did in Boromir whom he loved best.

Now how is that possible? How can the blood of Westernesse run through Faramir and not Boromir? It's impossible! It just doesn't make sense... Or does it?